I've been experiencing this same problem for a while and last time I fixed it I felt like it was a fluke and would recur, and it did. The solution described in this thread also doesn't seem to work for me.

Also I don't know if this is linked or not but I used to be able to send mail from squirrelmail web interface and now this doesn't work either I however can send from mail client like Evolution;- from remote machine

Also I don't know if this is linked or not but I used to be able to send mail from squirrelmail web interface and now this doesn't work either I however can send from mail client like Evolution;- from remote machine

So, this problem persists for me, and appears to even be worse, as this server which previously had uptime averages of months, now locks up where I cannot even ssh in and even physically at the terminal cannot get it to respond, so that only pressing the reset button returns it to functionality.